1. Field of the Invention
The present invention relates, in general, to a novel opening switch device for interrupting the flow of high amperage currents such as those which occur in inductive energy storage systems. The opening switch includes a novel plasma focus device incorporating an auxiliary electrode through which the current to be interrupted flows. This current is interrupted due to current pinching caused by the plasma focus discharge.
2. Description of the Prior Art
The present requirements for pulsed power loads such as high-energy lasers and particle-beam accelerators are for instantaneous powers that far exceed the capabilities of most continuous-duty power systems. A practical alternative is the employment of energy storage and power conditioning networks that compress energy, i.e., that store energy over comparatively long periods of time and then discharge the energy in a time interval appropriate for the load. It is thus possible to obtain tremendous amplification of the power that is supplied to the load. Ideally, the discharge time should be independent of the time required to charge the storage element. Capacitive, inductive, chemical, and inertial energy storage mechanisms have all been considered as possible energy storage techniques.
The pulsed loads of interest involve discharge times of tens of microseconds or less such that capacitive and inductive energy storage are the only viable storage methods available. As the magnetic energy density, B.multidot.H/2, can be approximately 100 to 1,000 times greater than the electrostatic energy density, D.multidot.E/2, for realistic values of electric and magnetic fields, inductive energy storage is a far more practical method of storing energy. For large systems, inductive energy storage can represent a significant difference in physical size, system efficiency, and operational convenience. Thus inductive energy storage is an alternative technique for temporarily storing large quantities of electric energy, such as energy levels in excess of 100 kJ. The attractiveness of inductive energy storage has been further enhanced by recent improvements in the technology of large inertial energy storage devices, such as homopolar generators, which are almost ideal current sources for such inductive networks.
While the electrical circuitry used in inductive energy storage is admittedly simple, progress in the development of such systems has been limited by the requirement for an opening switch which can interrupt an inductive charging current in the range of tens of kiloamperes to perhaps megamperes.
FIG. 1 illustrates a typical circuit for an inductive energy storage system. In FIG. 1, a low-voltage, high current source 10, such as a homopolar generator, is connected in series with an inductor 12 having an inductance L through switches S.sub.1 and S.sub.2 to form a charging circuit 20. An output circuit 22 including a load 14 having an impedance Z.sub.L in series with switch S.sub.3 is joined to the charging circuit 20 across switch S.sub.1 at junctions 16 and 18. The system first operates to charge the inductor by causing the current to build up in the charging circuit 20. For efficient operation, the current passing through the inductor must be built up in a time less than the L/R time constant of the inductor. If switch S.sub.1 is initially closed, a current I will finally be established in the inductor sometime after the switch S.sub.2 is closed. The energy in the coil (1/2LI.sup.2) must then be transferred to the load 14 in the form of a pulse. This occurs when switch S.sub.3 is closed and switch S.sub.1 is simultaneously opened. At that instant the peak output voltage will approach IR.sub.L for the load 14 where a resistive load is assumed such that Z.sub.L=R.sub.L. Eventually the voltage pulse will decay with a time constant of L/R.sub.L. It is the last state, namely the opening of switch S.sub.1, that represents the greatest obstacle in the operation of this circuit.
Numerous circuit analyses of inductive energy storage systems have been conducted. The work of Trost et al repeated in the Proceedings Of The First International Pulsed Power Conference, IEEE Pub. No. 76 CH 1147-8REG05, November 1976 is particularly pertinent to the present discussion. In FIG. 2, the results of computations by Trost et al are shown of the temporal characteristics of a voltage pulse applied to a resistive load. The resistance, R.sub.S, of the opening switch S.sub.1 is assumed to increase linearly with time such that R.sub.S =kt. In this analysis the "opening time" is defined to be the time necessary for the switch resistance to equal the load resistance. In this figure the normalized output voltage across the load is shown as k varies from R.sub.L /.tau. to 100 R.sub.L /.tau.. It should be noted that the peak voltage across the load increases as the switching time is reduced relative to L/R.sub.L. For this example, approximately 82 percent of IR.sub.L will appear across R.sub.L for k=100 R.sub.L /.tau.. This clearly depicts the significance of rapidly interrupting the current in the charging branch. The remainder of the pulse is a simple exponential decay with a time constant equal to L/R.sub.L.
Although commercial interrupters, such as vacuum bottles, exist for 60-Hz applications, these devices are not usually adequate for QUASI-DC since the current does not go to zero periodically. Moreover, typical applications for the switches necessary for use in inductive energy storage systems involve currents in the tens to hundreds of kiloamperes and load voltages of hundreds of kilovolts to perhaps megavolts. The problem is often compounded by the need for repetitive switching.
The requirements for an opening switch that would have a significant impact on the state-of-the-art in pulsed inductive energy storage are:
(1) Fast opening time (much less than L/R.sub.L); PA1 (2) Low loss in the closed state; PA1 (3) High impedance in the open state relative to the impedance of the load; PA1 (4) High current capabilities without significant erosion; PA1 (5) High restrike voltage with a rapid recovery rate; PA1 (6) A nondestructive switch that can be repetitively pulsed. PA1 (1) the inductance of the device is limited to values of tens of nanohenries which severly restricts the magnetic energy stored; PA1 (2) the time available to charge the inductor is limited to approximately 1 to 10 microseconds; and PA1 (3) a maximum of approximately 50 percent of the discharge current can be interrupted. PA1 (1) Fast opening time; PA1 (2) Capability for repetitive pulse operation; PA1 (3) Nondestructive operation; PA1 (4) Low loss in the closed state; PA1 (5) No limitation on time in the closed state; and PA1 (6) Capability for triggering other devices.
Several devices are currently being investigated as opening switches, and they can be divided into two general groups. The first group employs a mechanical disruption of the conductors such as by physically separating the electrodes (circuit breaker) or by cutting the conductors with explosives. The second group relies on some mechanism to increase the resistivity of the medium, either by heating or by the use of magnetic or electric fields. A major disadvantage of these prior art methods is that a single switch usually is not able to provide a sufficiently large change in resistance to satisfy the needs of inductive storage systems. For many of these switches, such as fuses, th conduction time is also rather limited. Also, many of these devices are incapable of providing reasonable repetition rates or in fact any repetition at all, as in the case of the fuse or explosive type switches. The solution to many of these deficiencies has been the construction of arrays of switching elements; however, this obviously increases the complexity and cost of the switching devices.
In recent years plasma devices have been considered as possible opening switches. One particularly interesting device is called a plasma focus. A plasma focus device is a coaxial, plasma accelerator in which magnetic energy stored in the coaxial geometry and internal circuit of the device is rapidly converted to plasma energy as an azimuthally symmetric current sheath collapses to form a densely compressed focused plasma. The collapse, which can be considered as a two-dimensional z-pinch, produces a hot, dense plasma with a radius on the order of 1 mm. At peak compression, an anomalous resistivity occurs in the plasma as indicated by an abrupt decrease in the plasma focus current. Decreases in the current by as much as 50 percent can occur in time intervals of tens of nanoseconds. Since the peak discharge current in a rather modest size plasma focus (e.g., 34 kJ) can easily exceed 600 kA, this represents a potentially exciting device for use as an interrupting switch. It is also important to note that bursts of electrons and ions with energies as large as 1 MeV are produced in the hot dense plasma coincident with the sharp decrease in plasma focus current. The particles stream out along the axis of the system are believed to be accelerated by strong electric fields generated in the hot plasma. Electron bursts with currents exceeding 30 kA have been reported.
Several researchers have recognized the potential of the plasma focus device as an interrupting switch in configurations for which the energy is stored as magnetic energy in the focus or in an external inductance which is inserted in series with the capacitors which drive the device. There are three fundamental limitations to this rather direct approach:
The second limitation is perhaps the greatest obstacle as this limits the maximum energy compression that can be achieved.
The inventors of the present invention have developed an inductive energy storage system utilizing plasma focus theory which does not suffer from the above discussed limitations. A novel modified plasma focus device is utilized in the present invention to provide the interrupting capability, while maintaining the inductive energy storage circuit separate from the switching means. This enables the charging time to be independent of the interrupting characteristics. Thus large energy compression ratios can be achieved.
Applications for the invention other than in inductive energy storage circuits are also feasible. There are currently applications in the electrical utility industry for a switch to interrupt currents in high-voltage dc transmission circuits. Such currents are particularly difficult to interrupt by conventional means as the current never goes through a natural current zero. The switch would be utilized in such applications by placing the plasma focus switch in series with the high-voltage transmission line at the location where it is desired to interrupt the current. This would entail connecting to points 16 and 18 in FIG. 1 and omitting the inductive charging circuit 20 and the load circuit 22.